A gas turbine engine typically includes a fan section, a compressor section, a combustor section and a turbine section. Air entering the compressor section is compressed and delivered into the combustion section, where it is mixed with fuel and ignited to generate a high-speed exhaust gas flow. The high-energy exhaust gas flow expands through the turbine section to drive the compressor and the fan section. The compressor section typically includes low and high pressure compressors, and the turbine section includes low and high pressure turbines. The high pressure turbine drives the high pressure compressor through an outer shaft to form a high spool, and the low pressure turbine drives the low pressure compressor through an inner shaft to form a low spool. The fan section may also be driven by the low spool inner shaft, either directly or indirectly through a fan drive gear system.
Typical gas turbine engines are designed such that peak operational efficiency occurs when the engine is operated during one or both of takeoff or top of climb (alternately referred to as climb out) conditions. During these conditions, the gas turbine engine utilizes maximum amounts of thrust output of all operational modes. The efficiency designs impact the size of the engine components and the temperatures at which the engine components run during each phase of engine operations. Since the typical gas turbine engine is designed for peak efficiency during takeoff or top of climb, where the turbine inlet temperature approaches its maximum allowable limit for highest efficiency and thrust, the gas turbine engine is operated at lower efficiencies during other modes, such as cruise, where the turbine inlet temperature is below the maximum allowable limit. Hybrid engine designs may be employed to enhance engine operation while operating at otherwise lower efficiencies.